I did this design for the Part 15 Longwave (1750M: 160KHz -- 190KHz) band. There is not a whole lot of commercial rigs that can cover this band, and the general coverage receivers that do don't cover it very well. The twin problems are a restriction of 1.0W of DC power into the final, and an antenna height restriction of 15M. The latter makes for a very short vertical. Such antennae are EEEEENORMOUSLY reactive with miniscule radiation resistances. Even with a highly efficient Class E power final, getting out 5.0mW is quite an accomplishment indeed.

General coverage receivers weren't designed to dig the typical "LOWFER" signal out of the muck. More powerful NDB stations, but not Part 15 signals. That leaves few alternatives but to DiY. To that end, this is a dedicated design that tunes across the 1750M band. This makes optimization possible. This is a necessity due to the nature of the signals you're trying to receive.

To that end, the passive doubly balanced mixer was chosen. This design isn't easily overdriven, is significantly quieter than either singly balanced, or unbalanced mixers. The only drawbacks are gain loss (make that up with subsequent amplification) and a rather high power requirement. Since Vf= 0.7V for a Si diode, the minimum power required is 4.9mW (6.9dbm). It works better with higher LO power: improved noise figure and higher gain (less loss actually). About 20mW represents the point of diminishing returns. Since the passive DBM works much better if all its ports are properly matched, a -3db resistive pad is included. The LO strip puts out 40mW (20mW + 20mW across the resistive pad).

The front end includes a Butterworth derived BPF to keep AM BCB out of the mixer. The audio output includes a third order Butterworth LPF. This sets an upper frequency of 3.18KHz. A CW filter (fc= 830Hz; Q= 10) is included for noise reduction and improved clarity. This filter can be switched out while searching for a signal, and switched in once one has been found. This requires a stable LO that won't drift out of the filter passband.

The LO is based on the series tuned Colpitts that was popular back in the 1950s. This topology allows for a high-Q tuner with a large L and small C. At these low frequencies, that's important since variable capacitors aren't all that large. By running it into very light loading, it is load isolated from varying loads that cause frequency pulling. Spectral purity is improved by the use of a tuned buffer, and a Pi-L matching network to match impedances between the 625R output impedance of the final and the 50R impedance of the DBM IF port. The oscillator shows some start-up drift due to the warming of the ferrite core the tuner inductor is wound. This takes place slowly enough, and soon settles down so that it is not an inconvenience.

The first audio pre was designed to match the DBM IF port. This being a grounded base design biased for an input impedance of 50R. It still manages a respectable Av= 30V/V. This stage is isolated from the DC rail by means of an active decoupler. This is important, given the high gain of the audio strip. More care than usual is required if audio frequency oscillations are to be avoided. The audio final is a conventional design, and puts out 1.5W -- sufficient for driving the speaker, a car radio replacement designed for 3.0W.

Front End
LO Strip
Audio Strip

Hey Dor,

I am NOT a LOWFER expert,but I'll chime in with a bit:

I have found a number of commercial receivers that are fully "spec'ed" at 150 or 100 Kc ( with some as low as 30 Kc) .

Many LOWFERS use R-75's and they seem to like them.My old R-70 (RIP) had "decent" sensitivity well below 100 Kc and seemed fully rockin' and rollin' above 100 Kc. My current R-71a doesn't dip down quite as low as the R-70 did, but is at good spec at 60 Kc and seemingly full spec at 100 Kc.

The problems for LOWFER reception as I see them may not be so much in the lack of good receivers ( commercial OR home brew),but simply due to the "nature of the beast". LOFWERS as you know, are usually very weak and the band is noisy both locally and due to lightning.

Most folks can't put up a really good receive antenna for such a low frequency either.I have set up LARGE antennas in remote QUIET locations a number of times over many years and may have heard only 1 or 2 LOWFERS on CW. LF can be tough territory (try copying SAQ !).

As I understand it, a straight up CW QSO is almost NEVER done down there."Noise evading" digital modes are the norm and it's through these I believe that 90 % the dx QSO'ing is done.I guess some folks also run conventional CW beacons but I am not sure how successful they are, even to a receiver on a big antenna.

So I just figured I give my OPINIONS. I certainly would NEVER discourage anyone from from trying anything home brew .

I think you would get much better feedback (than mine) on the LWCA website.

N1NQC

Ive used a TS-940 and TS-950SD to listen down there preceeded by a passive bandpass filter. With a full load of sharp filters and the usual Kenwood bells and whistles they has ID'ed several beacons.
Ive also used a 60's National HR0-500 with the LF-10 active and tuneable preselector since the late 70's.

Antennas have been various Beverages to 2000' but even 500-750' shows excellent directivity on the LW BCB. Those antennas are/were primarily for 160-40M ham bands but the past few years Ive done a lot of listening on 600M.

Carl